Date Awarded

1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

William G. MacIntyre

Abstract

This dissertation describes several studies of groundwater quality, the fate and transport of nutrients and atrazine in groundwater systems and physical processes at the sediment-water interface which impact on groundwater discharge. Groundwater monitoring wells were installed at the Goodwins Island and Catlett Island NERRS sites in Virginia and agricultural, suburban and forest land use sites in the York and James River Basins. Groundwater was analyzed for nutrients, pH, salinity and trace organics. Shoreline land use, associated nutrient loading and an estimate of total groundwater flux suggest that groundwater contributes up to 30% of the total nitrogen input to the Chesapeake Bay. Batch isotherm K&\sb{lcub}\rm d{rcub}& values for &\sp{14}&C-labelled atrazine sorption to low organic carbon (OC) aquifer solids ranged from 0.08 to 0.61 L/kg, much lower than for soil and other, higher carbon, solids. Kd was dependent on the surface area (SA) and surface iron (SFe), and can be described by:&&\rm K\sb{lcub}d{rcub} = 0.046(SA)+ 0.61(SFe) + 0.83&&. Kinetics of atrazine sorption onto low OC sediments showed an initial rapid reaction accounting for &>&95% of sorption within a few minutes at about 2 &\mu&g kg&\sp{lcub}-1{rcub}& min&\sp{lcub}-1{rcub}& followed by a slower (0.003 &\mu&g kg&\sp{lcub}-1{rcub}& min&\sp{lcub}-1{rcub}&) reaction. Comparison of homogeneous and heterogeneous atrazine hydrolysis reaction rates at elevated temperature showed no evidence for catalysis by aquifer solids. A physical mechanism for passive ventilation of relict biological structures in the benthic boundary layer is presented, a mathematical model of circulation in surrounding sediments was solved analytically and MODFLOW used to numerically determine induced hydraulic gradients and flow. Hydraulic head in surrounding sediments can be reduced by several centimeters in a zone extending over several hundred cm&\sp2&, and pore velocities of &>&10&\sp{lcub}-3{rcub}& cm/s and flux rates of tens to hundreds of mL/hr induced. Flume and field studies showed that large variability and poor repeatability of seepage meter measurements may be due to surface water flow across the meter. Pressure gradients in the boundary layer reduce hydraulic head within the meter and induce augmented seepage flow. Covering the seepage meter collection bag to isolate it from flow can reduce or eliminate this error.

DOI

https://dx.doi.org/doi:10.25773/v5-phza-mm35

Rights

© The Author

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